Human spinal disc prosthesis

ABSTRACT

The invention relates to a spinal disc endoprosthesis. The endoprosthesis has a resilient body formed of one or more materials which may vary in stiffness from a relatively stiff exterior annular gasket portion to a relatively supple central nucleus portion. Concaval-convex elements at least partly surround that nucleus portion so as to retain the nucleus portion and gasket between adjacent vertebral bodies in a patient&#39;s spine. Assemblies of endoprosthetic discs, endoprosthetic vertebral bodies, and endoprosthetic longitudinal ligaments may be constructed. To implant this endoprosthesis assembly, information is obtained regarding the size, shape, and nature of a patient&#39;s damaged spine. Thereafter, one or more prosthetic vertebral bodies and disc units are constructed in conformity with that information. Finally, the completed and conformed vertebral body and disc assembly is implanted in the patient&#39;s spine.

CROSS-REFERENCE

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/776,394 entitled “HUMAN SPINAL DISC PROSTHESIS” filed onFeb. 2, 2001, which is a reissue application of the U.S. Pat. No.5,865,846 entitled “HUMAN SPINAL DISC PROSTHESIS” filed on May 15, 1997,each of which is hereby incorporated by reference.

BACKGROUND

[0002] This invention relates generally to human prostheses, andespecially to spinal column vertebral disc prostheses. The inventionalso relates to surgical procedures for preparing the patient to receivea vertebral disc endoprosthesis, and for implanting that endoprosthesisin the patient's spine.

[0003] The herniation of a spinal disc and the often resultant symptomsof intractable pain, weakness, sensory loss, incontinence andprogressive arthritis are among the most common of debilitatingprocesses affecting mankind. If a patient's condition does not improveafter conservative treatment, and if clear physical evidence of nerveroot or spinal cord compression is apparent, and if correlatingradiographic studies (i.e., MRI or CT imaging or myelography) confirmthe condition, surgical removal of the herniated disc may be indicated.The process of discectomy—as the name implies—involves the simpleremoval of the disc without attempt to replace or repair themalfunctioning unit. In the United States in 1985, over 250,000 suchoperations were performed in the lumbar spine and in the cervical spine.

[0004] Statistics suggest that present surgical techniques are likely toresult in short-term relief, but will not prevent the progressivedeterioration of the patient's condition in the long run. Through betterpre-operative procedures and diagnostic studies, long-term patientresults have improved somewhat. But it has become clear that unless theremoved disc is replaced or the spine is otherwise properly supported,further degeneration of the patient's condition will almost certainlyoccur.

[0005] In the mid-1950's and 60's, Cloward and Smith & Robinsonpopularized anterior surgical approaches to the cervical spine for thetreatment of cervical degenerative disc disease and related disorders ofthe vertebrae, spinal cord and nerve root; these surgeries involved discremoval followed by interbody fusion with a bone graft. It was noted byRobinson (Robinson, R. A.: The Results of Anterior Interbody Fusion ofthe Cervical Spine, J. Bone Joint Surg., 440A: 1569-1586, 1962) thatafter surgical fusion, osteophyte (bone spur) reabsorption at the fusedsegment might take place. However, it has become increasingly apparentthat unfused vertebral segments at the levels above and below the fusedsegment degenerate at accelerated rates as a direct result of thisfusion. This has led some surgeons to perform discectomy alone, withoutfusion, by a posterior approach in the neck of some patients. However,as has occurred in surgeries involving the lower back where discectomywithout fusion is more common as the initial treatment for discherniation syndromes, progressive degeneration at the level of discexcision is the rule rather than the exception. Premature degenerativedisc disease at the level above and below the excised disc can and doesoccur.

[0006] Spine surgery occasionally involves fusion of the spine segments.In addition to the problems created by disc herniation, traumatic,malignant, infectious and degenerative syndromes of the spine can betreated by fusion. Other procedures can include bone grafts and heavyduty metallic rods, hooks, plates and screws being appended to thepatient's anatomy; often they are rigidly and internally fixed. Noneprovide for a patient's return to near-normal functioning. Though theseprocedures may solve a short-term problem, they can cause other, longerterm, problems.

[0007] A number of attempts have been made to solve some of the problemsdescribed above by providing a patient with spinal disc prostheses, orartificial discs of one sort or another. For example, Steffee, U.S. Pat.No. 5,031,437, describes a spinal disc prosthesis having upper and lowerrigid flat plates and a flat elastomeric core sandwiched between theplates. Frey et al., U.S. Pat. Nos. 4,917,704 and 4,955,908, discloseintervertebral prostheses, but the prostheses are described as solidbodies.

[0008] U.S. Pat. Nos. 4,911,718 and 5,171,281 disclose resilient discspacers, but no inter-connective or containing planes or like elementsare suggested, and sealing the entire unit is not taught.

[0009] It is the primary aim of the present invention to provide avertebral disc endoprosthesis which will perform effectively andefficiently within a patient's spine over a long period of time, andwhich will not encourage degeneration of or cause damage to adjacentnatural disc parts.

[0010] It is another object to provide a vertebral disc endoprosthesiswhich does not require pins or other common mechanical hinge elements,yet which permits natural motion of the prosthetic parts and theadjacent natural anatomy.

[0011] It is a related objective to provide a new vertebral discendoprosthesis surgical procedure which will decrease post-operativerecovery time and inhibit post-operative disc, vertebral body and spinaljoint degeneration.

[0012] It is yet another object to provide a method of installing theendoprosthesis so as to accurately mate the endoprosthesis with anadjacent specifically formed bone surface. An associated object is toprovide an endoprosthesis which will encourage bone attachment to, andgrowth upon, adjacent outer surfaces of the endoprosthesis.

[0013] Yet another object is to provide a vertebral endoprosthesis inwhich the parts are non-oncogenic.

[0014] Still another object is to provide a vertebral discendoprosthesis having a resilient element to accommodate shocks andother forces applied to the spine.

[0015] Another object is to provide a highly effective vertebralendoprosthesis which includes several disc endoprostheses and one ormore prosthetic vertebral bodies. A related object is to provide theseelements in a pre-assembled array for implantation in a patient.

SUMMARY OF THE INVENTION

[0016] To accomplish these objects, the invention comprises a resilientbody formed of a material varying in stiffness from a relatively stiffexterior portion to a relatively supple central portion. Aconcaval-convex means at least partly surrounds that resilient body soas to retain the resilient body between adjacent vertebral bodies of apatient's spine. If medical considerations so indicate, several discendoprostheses can be combined with one or more endoprosthetic vertebralbodies in an entire assembly.

[0017] To implant this endoprosthesis assembly, information is obtainedregarding the size, shape, and nature of a patient's damaged naturalspinal discs. If one or more of the patient's vertebral bodies alsorequire replacement, information about those bodies is also obtained.Thereafter, one or more prosthetic disc units and interposed prostheticvertebral body units are constructed and preassembled in conformity withthat information. Finally, the completed and conformed prosthetic discand vertebral body assembly is implanted in the patient's spine.

[0018] Other objects and advantages of the invention will becomeapparent upon reading the following detailed description and uponreference to the drawings. Throughout the drawings, like referencenumerals refer to like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a fragmentary vertical view of a portion of a humanspine in which is installed a novel vertebral disc endoprosthesisembodying the present invention;

[0020]FIG. 2 is a fragmentary side elevational view similar to FIG. 1showing the elements of a patient's spine and having a novel vertebraldisc endoprosthesis embodying the present invention installed therein;

[0021]FIG. 3 is a sectional view taken substantially in the plane ofline 3-3 in FIG. 1;

[0022]FIG. 4 is an exploded view of the novel vertebral discendoprosthesis;

[0023]FIG. 5 is a vertical fragmentary view of a patient's spine similarto FIG. 1, but showing a series of novel disc endoprosthesis unitsinstalled in the spine and interconnected to one another;

[0024]FIG. 6 is a fragmentary sectional view of a patient's spinesimilar to FIG. 3 and taken along line 6-6 in FIG. 5, but showing anatural upper vertebral body, and upper endoprosthetic disc; an adjacentendoprosthetic vertebral body; a second or lower endoprosthetic disc;and a second or lower natural vertebral body; p FIG. 7 is a sectionalview taken substantially in the plane of line 7-7 of FIG. 6;

[0025]FIG. 8 is a fragmentary side elevational view of the assemblyshown in FIG. 6; and

[0026]FIG. 9 is a fragment vertical view, similar to FIG. 1, of aportion of a human spine in which is installed a variant form of thenovel vertebral disc endoprosthesis the variant form having a prostheticlongitudinal ligament;

[0027]FIG. 10 is a sectional view taken substantially in the plane ofline 10-10 in FIG. 9;

[0028]FIG. 11 is a top view of a retainer means for use with a vertebraldisc endoprosthesis;

[0029]FIG. 12 is a sectional view taken substantially in the plane ofline 12-12 of FIG. 11;

[0030]FIG. 13 is a side view of a vertebral disc endoprosthesis having agroove for receiving the retainer means; and

[0031]FIG. 14 is a cross-sectional view of the retainer means in use.

DETAILED DESCRIPTION

[0032] While the invention will be described in connection with apreferred embodiment and procedure, it will be understood that it is notintended to limit the invention to this embodiment or procedure. On thecontrary, it is intended to cover all alternatives, modifications, andequivalents as may be included within the spirit and scope of theinvention as defined by the appended claims.

[0033] Turning more specifically to FIGS. 1-3, a portion of a humanspine 10 is shown. The illustrated spine 10 has been subjected to adiscectomy surgical process. To discourage degeneration of or damage tothe natural vertebral bodies 12 and 14 and their respective facetjoints, in accordance with the invention, a vertebral discendoprosthesis 18 is affixed between the adjacent natural vertebralbodies 12 and 14. Here this vertebral disc endoprosthesis 18 comprises aresilient disc body 20 having a relatively stiff annular gasket exteriorportion 22 and a relatively supple nuclear central portion 24. Theannular gasket 22 can be formed from a suitable biocompatible elastomerof approximately 90 durometer hardness and the nuclear central portion24 can be formed from a softer biocompatible elastomeric polymer ofapproximately 30 durometer hardness.

[0034] Concaval-convex means 30 surround the resilient body 20 to retainthe resilient body 20 between the adjacent natural vertebral bodies 12,14 in a patient's spine 10. To this end, as shown in FIG. 3, theconcaval-convex means 30 comprise two generally L-shaped supports 32 and34. The supports 32, 34 each have confronting first concaval-convex legs42, 44, each leg being of relatively constant cross-sectional thickness.Each leg 42, 44 has an outer convex surface 52, 54 for engaging theadjacent bone of the natural vertebral bodies 12, 14. Correspondinginner concave surfaces 62, 64 in confronting array retain the resilientbody 20 in its illustrated compressive force shock-absorbing position.These supports 32 and 34 can undergo principle movement away from oneanother, but only limited secondary translational, rotational anddistractional motion will occur. Each support 32, 34 has a second wingor leg 72, 74 extending generally perpendicularly to the first legs 42,44 respectively, and adapted for affixation to the adjacent bonestructure. To carry out aspects of the invention described below, thisaffixation is effectively accomplished by cannulated screw devices 82,84 which may be of a biodegradable type manufactured by Zimmer of Largo,Fla. Each device 82, 84 comprises a screw 92, 94; and a screw anchor102, 104 adapted to threadably receive the screw extends radially intoand seats within the bone structure 12, 14 as especially shown in FIG.3.

[0035] To discourage and prohibit migration of fluids between theendoprosthesis 18 and adjacent parts of the anatomy, a seal member 110is attached to the supports 32, 34 so as to surround the resilient body20 comprised of the gasket 22 and nucleus 24, in accordance with anotheraspect of the invention. Here, this seal member 110 comprises a flexiblesheet material having a multiplicity of pores. Preferably, the pores arefrom about 5 microns to about 60 microns in size. A flexible, strongpolymer sheet material from which this seal is formed can be aKevlar-like material, or it can be Goretex-like material, or otherappropriate biocompatible material, such as polyether, polyurethane, orpolycarbonate urethane membranes, can be used. Kevlar material isoffered by the E.I. DuPont de Nemours Company of Wilmington, Del. andGoretex material is offered by the W.T. Gore Company of Flagstaff andPhoenix, Ariz. Known sealing material can be applied to the flexiblesheet material so as to render the flexible sheet material substantiallyimpervious to the passage of any fluid. A watertight seal is perfectedwhen the seal 110 is glued or otherwise affixed to the legs 42, 44 andmediate portions of the legs 72, 74 as suggested in FIGS. 1-3.

[0036] In an alternative embodiment, the watertight seal between theendoprosthesis 18 and adjacent parts of the anatomy can be provided bydeveloping a groove 402 completely encircling the periphery of each ofthe legs 42, 44. Only one of the grooves is shown in FIG. 13. In thisembodiment, the seal member 410 is provided with a beaded edge 412 foreach groove. Additionally, a retaining band 415 is provided for eachgroove to retain the seal member 410 in grooves 402. The retaining bands415 can be in the form of a biocompatible monofilament wire of, forexample, stainless steel or titanium, a synthetic polymer cable or abraided wire cable. As shown in FIG. 11, each retaining band is crimpedanteriorly by a crimping sleeve 420. Of course, more than one crimpingsleeve may be used, if necessary. Although one sealing arrangementconsisting of the groove, beaded edge and retaining band is shown inFIG. 14, it should be understood that the sealing arrangement on theconcaval-convex leg of the other support is identical in design andfunction.

[0037] In use, the seal member 410 is placed about the concaval-convexmeans 30. The retaining bands 415 are then placed adjacent to therespective groove 402 and crimped anteriorly, thereby fitting the bandsinto the grooves. Each beaded edge 412 prevents the slipping of the sealmember underneath the retaining band. Thus, the retaining band, thegroove and the beaded edge all cooperate to provide a water-tight sealto prevent the migration of fluids between the endoprosthesis 18 andadjacent parts of the anatomy. Glue can also be used to affix the sealmember to the concaval-convex means 30 as a supplemental means forperfecting the seal.

[0038] In accordance with another aspect of the invention, the supports32, 34 are formed of a biocompatible metal which may contain chromiumcobalt or titanium. Surface roughening or titanium beading 112, 114 onthe exterior surfaces 52, 54 of legs 42, 44 encourages positive bondingbetween the adjacent bone and the convex surfaces 52, 54.

[0039] As suggested in FIGS. 9 and 10, a prosthetic longitudinalligament 250 can be connected between the screws 92, 94 to limit motionsbetween elements of the spine 10 in the area where the endoprosthesis 18is implanted. This strap 250 may be made of the Kevlar-like material orthe Goretex-like material described above, or it may be made of anyother strong biocompatible material.

[0040] In accordance with another aspect of the invention, multipleendoprosthetic disc units can be placed in series with a straddlinginterlock appendage providing stability and fixation as shown in FIG. 5.Entire portions of a patient's spine can be replaced by a series ofinterconnected endoprosthetic vertebral bodies and endoprosthetic discunits. FIGS. 6-8 show an upper natural vertebral body unit 312 to whichan upper endoprosthetic body 308 has been attached. A lower naturalvertebral body 314 has attached, at its upper end, an endoprostheticdisc unit 318. Between these endoprosthetic disc units 308 and 318 is anendoprosthetic vertebral body 320. As suggested by FIG. 7, theendoprosthetic vertebral body 320 need not be irregularly shaped incross sectional aspect; rather, manufacturing processes may suggest thatit have a circular cross-sectional shape. As show in FIGS. 6 and 8, thisendoprosthetic vertebral body 320 comprises a titanium element 321, towhich are attached the preformed upper and lower endoprostheticvertebral body upper and lower concaval-convex elements 322, 324. Eachconcaval-convex element 322, 324 is attached to the prosthetic vertebralbody 320, as shown in FIG. 7, by extending set screws 330 through thetitanium vertebral body 321 into a stem-like projection 331 extendingfrom each of the concaval-convex elements 322, 324. A hole 360 in thebody 320 accommodates the stem-like projections 331 of theconcaval-convex elements 322 and 324. The stem-like projection 331 ofthe concaval-convex elements 322 and 324 is used only in conjunctionwith a prosthetic vertebral body implant construction 320.

[0041] An ear 340 is affixed, as by weldments 341, to a leg 342extending from a concaval-convex element 322 as illustrated in FIGS. 6and 8. An anchor 352 can be threaded into the endoprosthetic vertebralbody 320, and a screw 362 can be turned into the anchor 352 so as torigidly assemble the leg 342 to a leg 354 extending from the lowerendoprosthetic disc unit 318.

[0042] The upper disc endoprosthesis 308, the endoprosthetic vertebralbody 320, and the lower disc endoprosthesis 318 can all be assembled andinterconnected as a unit before implantation in a patient's body whenindicated.

[0043] As also suggested in FIG. 6, the annular corners 372, 374 ofnatural vertebral bodies 312, 314 each can extend irregularly radiallyoutwardly of the adjacent disc endoprosthesis 308, 318. However, thecorners 382B, 384B of the prosthetic vertebral body 320 do not generallyextend significantly outside those disc units 308, 318, thusdiscouraging vertebral body engagement with and consequent abrasion orother damage to adjacent portions of the patient's natural anatomy.Preferably the endoprosthetic vertebral body 320 is not exactly rightcylindrical in shape, but is rather slightly biconical; that is, theendoprosthetic vertebral body 320 has a waist 390 of minimum radius R atan axial medial point as suggested in FIG. 6.

[0044] According to yet another aspect of the invention, novel surgicalprocedures permit effective and permanent installation of theendoprosthetic vertebral body 320 and associated parts. First, a surgeonor medical technician develops information about the size, shape andnature of a patient's damaged vertebral body or bodies from radiographs,CT and/or MRI scans, noting specifically the anterior-posterior andlateral dimensions of the end plate of each involved vertebral body andthe vertical height of the anterior aspect of each involved vertebraland/or proximate vertebral body and vertical height of the mid portionof involved and proximate relatively normal intervertebral disc spaces.This information is transmitted by telephone, computer datalink ordocumentary transport to a specialized laboratory. That laboratoryconstructs one or more prosthetic assemblies of the sort shown in FIG. 6in conformity with the received information and this disclosure. Each ofthe assemblies can include a prosthetic vertebral body 321, and at eachbody end is a prosthetic disc 308, 318. Each prosthetic disc unitcomprises, in turn, the concaval-convex elements 30; the resilient body20 interposed between the concaval-convex elements; and the seal unit110 secured around the interior legs and resilient body. Thereafter, thecompleted and conformed assembly is implanted in the patient's spine 10.

[0045] When the unit or units have been received and the patientproperly prepared, the damaged natural spinal disc or discs andvertebral body or bodies are removed and the adjacent spinal bonesurfaces are milled or otherwise formed to provide concave surfaces toreceive the confronting convex surfaces 52, 54. Thereafter, the discunits and vertebral body are installed in the patient's spine.

[0046] To accurately locate the concaval-convex surfaces in thepatient's spine, holes 382A, 384A (FIG. 3) are precisely located andthen formed in the bone structure using a measuring instrument centeredin the evacuated natural intravertebral disc space. These holes are thentapped to form female threads therein. When the threads have beenformed, the anchors 102, 104 are implanted in the respective tappedholes, thereby creating reference points located precisely with respectto the patient's spine. After the holes have been formed and the anchors102, 104 implanted, a bone surface milling jig (not shown) is affixed tothe anchors 102, 104 and the desired concave surfaces of predeterminedshape are formed on the inferior and superior surfaces of the opposingvertebral bodies using one of a selection of predetermined milling heador bit sizes. Thereafter, the bone milling jig is removed and theconcaval-convex elements 52, 54 identical in shape to the milledsurfaces 112, 114 are inserted between the distracted milled vertebralbodies 12, 14. The distraction device is then moved. The concaval-convexstructures are then attached by the same anchors 102, 104 to the bone,thus insuring a precise and stable mate between the bone surfaces andthe convex surfaces 52, 54.

[0047] If necessary, a damaged implanted nucleus and/or gasket 24 can beremoved and replaced. This can be accomplished by slitting the seal 110;removing the annular gasket 24 and damaged nucleus 22, and replacingthem with new, undamaged elements. Thereafter, the seal 110 can bere-established by suturing or gluing closed the slit seal.

We claim:
 1. A method of surgery comprising: forming concave surfaces inendplates of confronting vertebral bodies; and inserting between theformed concave surfaces an intervertebral disc endoprosthesis whereinthe intervertebral disc endoprosthesis comprises: L-shaped supportswherein each of the L-shaped support comprises an exterior convexsurface adapted to mate with one of the formed concave surfaces; and aresilient body interposed between the L-shaped supports.
 2. The methodof claim 1, further comprising affixing the L-shaped supports to theconfronting vertebral bodies.
 3. The method of claim 1, furthercomprising implanting at least one anchor in at least one of theconfronting vertebral bodies.
 4. The method of claim 3, wherein theimplanting is located in an anterior surface of the at least one of theconfronting vertebral bodies.
 5. The method of claim 4, furthercomprising affixing a bone surface milling mechanism to the at least oneanchor.
 6. The method of claim 1 wherein the resilient body comprises arelative stiff portion and a relative supple portion.
 7. A method ofsurgery comprising: implanting at least one anchor in an anteriorsurface of at least one of confronting vertebral bodies; forming concavesurfaces in the endplates of the confronting vertebral bodies; andinserting between the formed concave surfaces an intervertebral discendoprosthesis comprising: confronting supports, each support having anexterior convex surface adapted to mate with one of the formed concavesurfaces; and a resilient body interposed between the supports.
 8. Themethod of claim 7, further comprising affixing a bone surface millingmechanism to the at least one anchor.
 9. The method of claim 7, furthercomprising removing damaged disc material.
 10. The method of claim 7wherein the resilient body comprises a relative stiff portion and arelative supple portion.
 11. A method of spinal surgery comprising:forming mounting holes in one or more vertebral bodies of a patient'sspine; implanting at least one anchor into one of the mounting holes;utilizing the at least one anchor to mount a bone mill on the patient'sspine; milling confronting bone surfaces on and in the patient's spineto a predetermined surface shape; removing the bone mill; and mountingan intervertebral disc endoprosthesis having a predetermined outersurface shape so that outer surfaces of the intervertebral discendoprosthesis mate with the previously milled bone surfaces and arecapable of motion relative to each other.
 12. A method of endoprostheticdiscectomy surgery comprising: receiving information about the size,shape, and nature of a patient's involved natural spinal vertebralbodies and natural spinal vertebral discs from imaging devices; removingat least the involved and damaged natural spinal disc material from thepatient's spine; implanting at least one anchor into a hole having apredetermined position in an anterior surface of at least one adjacentvertebral body; forming concave surfaces in the adjacent vertebralbodies; and implanting into the patient's spine, an intervertebral discendoprosthesis comprising a resilient disc body and concaval-convexelements that at least partly surround and are capable of movementrelative to the resilient disc body in the patient's spine.
 13. Themethod of claim 12, further comprising affixing a bone surface millingmechanism to the at least one anchor.
 14. The method of claim 12 whereinthe concaval-convex elements are adjacent to the resilient body.
 15. Themethod of claim 12 wherein the concaval-convex elements are in contactwith the resilient body.
 16. A method of surgery comprising: implantingat least one anchor into a hole having a predetermined position in ananterior surface of at least one adjacent vertebral body; affixing abone surface milling mechanism to the at least one anchor; formingconcave surfaces in the endplates of the adjacent vertebral bodies; andinserting between the formed concave surfaces an intervertebral discendoprosthesis, comprising: confronting concaval-convex supports, eachsupport having an exterior convex surface adapted to mate with one ofthe formed concave surfaces; and a resilient body between theconcaval-convex supports.
 17. The method of claim 14 wherein theconcaval-convex supports are adjacent to the resilient body.
 18. Themethod of claim 14 wherein the concaval-convex supports are in contactwith the resilient body.
 19. The method of claim 14 wherein theresilient body comprises a gasket portion and a nucleus portion.
 20. Amethod of inserting a prosthesis in a disc space between two adjacentvertebral bodies, comprising: implanting at least one anchor into a holehaving a predetermined position in an anterior surface of at least oneadjacent vertebral body; affixing a bone surface milling mechanism tothe at least one anchor; forming at least a portion of a hemisphericalcavity in an endplate of one of the vertebral bodies, the endplate havea remaining surface surrounding the cavity; and inserting anendoprosthesis into the disc space and the cavity, the endoprosthesisincluding at least one support having an exterior convex surface adaptedto mate with the cavity, and a body interposed between the at least onesupport and the second vertebral body, wherein the at least one supportis movable relative to the body.